Semiconductor lasers are commonly used in optical transceivers for telecommunications and data communication networks. The lasers used in such optical transceivers are commonly of the edge-emitting type. The edge-emitting laser of an optical transceiver is commonly coupled to the fiber with an aspheric lens or other discrete optical element because the light that the laser emits is not focalized or collimated, i.e., it diverges in a cone shape as it propagates. While the use of lenses to couple edge-emitting lasers to fibers in optical transceivers works reasonably well, it would be desirable to improve transceiver manufacturing economy by minimizing the number of transceiver parts and the attendant steps needed to achieve optical alignment among them.
Edge-emitting lasers for optical transceivers are fabricated on semiconductor wafers using standard photolithographic and epitaxial methods, diced into chips, and portions of each chip coated with highly-reflective and anti-reflective coatings. The finished chips can then be tested. It would be desirable to minimize the number of manufacturing steps as well as to enhance testability.
It has also been proposed to integrate a diffractive lens and an edge-emitting laser on the same chip. For example, U.S. Pat. No. 6,459,716 to Lo et al. discloses a device in which an edge-emitted beam produced by an edge-emitting laser is reflected by an angled surface toward a lower reflective surface that is parallel to the beam-emission direction and parallel to the chip surface, which, in turn, reflects the beam upwardly in a direction generally perpendicular to the chip surface. The upwardly reflected beam is then emitted through an aspheric lens formed in a material on the chip surface to collimate laser beam divergence. A transceiver having such a device can be manufactured more economically than one in which a separate lens is included. Nevertheless, the device is not straightforward to fabricate due to the inclusion of a waveguide to direct the beam from the laser toward the angled surface. Also, the geometry of the device may make its optical characteristics sensitive to wafer thickness errors.
Vertical Cavity Surface Emitting Lasers (VCSELs) are often preferred by end-users because of their high coupling efficiency with optical fibers without the need to provide beam shape correction, thus reducing test/packaging costs. VCSELs, however, still have problems with regard to single-mode yield control when manufactured for very high speed operation.
Efforts have also been made in the industry to convert an edge-emitting device into a vertical-emitting device. For example, U.S. Pat. No. 7,245,645 B2 discloses one or both of the laser facets etched at 45° angles to form a 45° mirror that reflects the laser beam vertically. In this solution, however, the 45° mirror is within the laser cavity. U.S. Pat. No. 5,671,243 discloses using conventional 90° laser facets, but outside of the lasing cavity there is a 45° reflection mirror that turns the beam towards in the direction of the surface. Nevertheless, the inclusion of an etched mirror inside or outside of the laser cavity requires high quality facet etching to be performed during fabrication. Performing high quality etching presents significant reliability issues, especially when performing dry etching under high operating power due to facet damage that can occur during the dry etching process.
U.S. Pat. No. 7,450,621 to the assignee of the present application discloses a solution that overcomes many of the aforementioned difficulties. This patent discloses a semiconductor device in which a diffractive lens is integrated with an edge-emitting laser on the same chip. The diffractive lens is monolithically integrated with the edge-emitting laser on an indium phosphide (InP) substrate material. The monolithic integration of a diffractive lens on the same chip in which the edge-emitting laser is integrated requires the performance of multiple Electron Beam Lithography (EBL) exposure and dry etching processes. It would therefore seem that the device fabrication costs would increase. However, with respect to using a separate lens to correct the beam divergence before the light enters the optical fiber, the overall cost of a monolithic integration of a laser with a diffractive lens is still less than the cost of packaging separate components.
In FIG. 1 of U.S. Pat. No. 7,450,621, the layer 20 (typically InP) in which the lens 18 is to be formed is grown on multiple quantum well (MQW) layers that have been etched to form the laser 10. Although the materials that are used for the MQW layers that form the laser 10 are not explicitly recited in the patent, for 1300-1550 nanometer (nm) range wavelength application, the MQW layers typically comprise Indium Gallium Aluminum Arsenide (InGaAlAs) due to their excellent temperature characteristics. Assuming InP is used for layer 20, before growth of the InP layer 20, the Al-containing MQW layers that make up the laser 10 have to be etched away. However, the Al-containing material is easily oxidized when exposed to humidity and oxygen (O2). The oxidized Al-containing material may not be completely removed during the etching process. At operating lasing conditions, any remaining oxidized Al-containing material can cause a portion of the injection current to leak through the interface between the InP and the InGaAlAs MQW, which can detrimentally affect device performance and reliability.
Another potential problem is the difficulty associated with using a wet chemical etching process to realize a reverse-mesa ridge structure having a low series resistance. Usually, such a reverse-mesa ridge structure is realized by selectively etching the InP layer down to an InGaAsP etch-stop layer. It can be difficult from a process standpoint to realize a reverse-mesa ridge structure that ends precisely at this interface using photolithographic and wet chemical etching techniques. In particular, if the reverse-mesa ridge structure extends over the interface, the wet chemical etching of the InP layer can destroy the MQW layers because under the InP layer there is no etch-stop layer. On the other hand, if the reverse-mesa ridge structure does not reach the interface, this can prevent the injection current from passing through all of the MQW layers, which can result in very large optical losses due to un-injected MQW layers. These difficulties can reduce manufacturing yield and increase costs.
It would be desirable to provide a semiconductor device in which an edge-emitting laser is integrated with a diffractive lens. It would also be desirable to provide such a semiconductor device that is reliable, economical to manufacture and that can be manufactured with high yield.